Irradiation of 2,5-dimethylphenacyl (DMP) esters (1a-c) in benzene or cyclohexane solutions produces the corresponding free carboxylic acids (2a-c) in high chemical yields, along with 6-methyl-1-indanone (3). In methanol, 2-(methoxymethyl)-5-methylacetophenone (4) is formed as a coproduct. Quantum yields for the photorelease of the DMP group are higher in nonpolar solvents, phi approximate to 0.2, than in methanol, phi approximate to 0.1. The photoreaction is initiated by efficient photoenolization, phi approximate to 1. Three transient intermediates were identified by laser flash photolysis of la, the triplet-state photoenol and the two ground-state photoenols of Z and E configuration. Release of the acids 2a-c occurs predominantly from the E isomer, which has a lifetime of about 2 ms in methanol and 0.5 s in benzene solution. The present data are essential for potential applications of the DMP moiety as a photoremovable protecting group in organic synthesis or biochemistry ("caged compounds").